1. Field of the Invention
This invention relates a zinc-cobalt-iron alloy electroplating bath solution and a zinc alloy plated product which is able to form a satisfied plated product having a black chromate film by treating with a chromate solution containing no silver.
2. Description of the Prior Art
As chromate treatment of zinc plating is extremely effective in improving corrosion resistance, it is being carried out in many fields. The types of chromate normally used are bright chromate having metal brightness, color chromate in which corrosion resist is particularly good, green chromate, black chromate, etc. but in the recent years, from the advantages of design and fashion, the demand for black chromate is increasing.
However, as black chromate was obtained traditionally by treating a zinc plated product with silver containing chromate solution made up by adding silver nitrate to mainly anhydrous chromic acid or the corrosion resist of the film being made poorer than color chromate due to the effects of silver being in the chromate film.
In order to resolve these problems, it is proposed that a zinc-iron alloy plating which co-deposits 0.2-0.8% (hereinafter indicated merely by "%") iron on zinc be used as base plating. By treating this alloy plating deposit with chromate solution not containing silver, it gives a good corrosion resist black chromate film. However, in this black chromate obtained by this method, compared to the chromate film formed by the ordinary chrome plating, it has the shortcoming of heat resist being slightly poorer and for parts being used in the engine room of the automobile, it would not be able meet the heat resist requirements.
In consideration of traditional technology concerning formation of black chromate film on zinc plated product having the problems explained above, the object of this invention is to provide a way to form a black chromate film excelling in corrosion resist, heat resist with a chromate solution not containing silver on a zinc plated product.